Embodiments of the present invention relate generally to wireless communication. More specifically, embodiments of the invention relate to systems and methods for transmitting signals from a wireless device where the number of transmit antennas is larger than or equal to the number of spatial streams transmitted.
Delay diversity is a technique used to increase the performance of a wireless link by transmitting delayed copies of the same input data signal from multiple transmit antennas. For example, wireless devices, such as wireless routers and other devices commonly used in various types of Wireless Local Area Networks (WLANs), utilize multiple transmit signals and multiple antennas for transmission of input data streams or signals. These multiple transmission channels can be used to transmit copies of the input data stream or data signal. The redundancy of transmitting copies of the data stream increases reliability of the transmit signals. However, to avoid interference between these copies, a delay or time shift is introduced to create a spatial spread between the transmitted copies.
More specifically, FIG. 1 shows a block diagram of a system in which delay diversity is used to create spatial spreading between transmitted signals. In this system 100, an input data stream 101 can be provided to Forward Error Correction (FEC) encoder 105. Codewords are created by FEC encoder 105 adding parity bits to the input data stream 101. The codewords created by FEC encoder 105 can be provided to puncture module 110 which selectively removes redundant bits to provide for more efficient transmissions. Puncture module 110 then supplies the codewords to spatial stream parser 115. Spatial stream parser 115 separates or copies the input data stream into a number (Nss) of spatial streams. Generally speaking, the number of spatial streams is less than or equal to the number of antennas 180 and 185 and/or the number (Ntx) of transmit streams.
The separate spatial streams can then be applied to frequency interleavers 120 and 130 which re-order or rearrange the bits prior to the streams being mapped to individual quadratture carriers by Quadruple Amplitude Modulation (QAM) mapping modules 125 and 135. A spatial spread can then be applied to the individual spatial streams by Walsh matrix operation (W) 140. With this technique, the Nss spatial stream input signals are first spread by a multiplication with the first Nss columns of the Ntx-by-Ntx Walsh matrix:
  W  =            [                                    1                                1                                1                                1                                                1                                              -              1                                            1                                              -              1                                                            1                                1                                              -              1                                                          -              1                                                            1                                              -              1                                                          -              1                                            1                              ]        .  
This scheme reduces to a conventional delay diversity system when there is only one spatial stream (Nss=1), because for that case only the first column of the Walsh matrix is used which is an all-ones column, so the ‘W’ operation reduces to copying one input stream to Ntx identical outputs.
After this operation, the transmit streams output by Walsh matrix operation 140 can be applied to Inverse Fast Fourier Transform (IFFT) modules 145 and 160 to combine the spatial streams and sub-carriers into a time domain signal. A different cyclic delay is applied to each of the transmit streams (Ntx) by one or more cyclic delay modules 165. A Guard Interval (GI) can be inserted between blocks of data on each transmit stream and the individual transmit streams by GI modules 150 and 170. Finally, the individual transmit streams can be used by analog and RF modules 155 and 175 to modulate a Radio Frequency (RF) carrier for transmission by antennas 180 and 185.
A disadvantage of such spatial spreading is that, for highly correlated channels, each spatial stream experiences deep nulls for every antenna orientation. This is because the transmitted signals for each spatial stream are delayed copies that cancel each other on certain frequencies. For Nss=1 and Ntx>1, at least one deep null in the received spectrum exists. For Nss<1 in strongly correlated channels, each spatial stream experiences at least one deep null in the received spectrum, but the nulls are generally at a different location for each spatial stream.
For at least the foregoing reasons, improved systems and methods are needed.